The disclosure refers generally to electric energy production systems. More particularly it refers to the installations of the type of electric energy production power plants feeding an electricity transmission network.
At present, the auxiliary equipment of an electric power plant is fed from an alternating current auxiliary network derived from the energy extraction network of the power plant, which itself is connected to the electricity transmission network fed by the power plant and to the energy production unit.
Generally, these installations also comprise an emergency direct current network which is used in case of loss of the main energy sources feeding the auxiliary network.
Furthermore, these installations comprise various devices that allow ensuring various network services through the power plant. The term “network service” refers to the technical functionalities carried out through the electricity production power plants in order to ensure proper operation of the electricity transmission network to which they are connected, as for example rapid supply of surplus electric energy that allows answering to the primary response demand that the power plant must be capable of supplying at the request of the network.
The application for European patent published under the reference EP 2595266 (A1) describes for this purpose an electric energy production installation comprising means for generation of electric energy meant to be connected to a distribution network and storage means for a part of the electric energy produced in the form of mechanical energy, through flywheels. The energy stored is restored on demand to the installation in electric form in order to ensure support functions such as:                frequency and voltage regulation (in permanent and transient regime);        support of small outages of the network;        the guarantee of an optimal load state of the storage means in cases of outage;        the use of energy stored in mechanical form to bear the shutdown of the main energy production unit and particularly limitation of the phenomenon of excess speed of the generator shaft;        support on restarting of the power plant in case of extended outage of the network.        
The electric energy production power plant thus described includes an electric energy production system, generated from the kinetic energy stored in the flywheels. This system is associated with the main electric energy production unit which provides electric power to the electric distribution network in order, on the one hand to compensate for the inertia of the steam turbine which drives the generator of the main production unit if required for the power plant to ensure the primary response requested by the network and, on the other hand, to provide electricity to the auxiliary network of this power plant.
This electric energy production system is managed by a controller controlling the operation of the energy storage means and their connection with the electricity generation means and transportation network.
The document cited above highlights the advantages procured by setting up an energy storage system in a power plant in order to improve its capacity to provide primary response, while it is operational. However, it is possible to envisage taking advantage of the functionalities procured by such a system when the power plant is shutdown and that the electric energy stops being supplied by the main unit. In fact, today several electric power plants have been led to operate in a cyclical manner, i.e. during a given period (season, day, week . . . ). They alternate between periods of operation and periods of shutdown. The term “shutdown” refers to periods during which the production unit of the power plant does not supply power to the transmission network, as the turbine is shutdown and the generator is connected or disconnected from the network. Consequently, the use during these periods of shutdown of the energy storage system will allow increasing the profitability of these power plants.
Furthermore, the cited document describes an energy storage system in the form of kinetic energy. However, it may be judicious to consider other types of energy storage systems. The electric energy storage and production through electrochemical generators can thus prove to be very competitive, in terms of cost, vis-à-vis storage in the form of mechanical energy, particularly when high autonomy is required. In fact, energy storage in kinetic form is very appropriate to provide high power for short time (to the order of the minute), but it is less appropriate for longer periods (to the order of several minutes to several hours).
Also some auxiliaries, currently called critical auxiliaries, of an electric power plant require to be fed for periods attaining several hours, in case of break down, i.e., inoperability, of the normal power supply source of the auxiliary network of the power plant. For example, this is the case of shaft line lubrication systems which, in case of break down of the auxiliary network power supply of the power plant, must be maintained operational in order to ensure its shutdown and restarting under good conditions.
In fact, the power supply of the auxiliaries of an electric power plant is generally carried out, as illustrated in FIG. 1, through two types of distribution networks:                one main auxiliary network, average voltage 4 and low voltage 15, of type having alternating current, the main function of this network being to feed, from the energy transmission network 1 or the main energy production unit 2, all the medium voltage 6 and low voltage 16 auxiliaries of the power plant required for its proper operation. In these auxiliaries, one typically finds electric pump units, actuators as well as auxiliary motors operating on medium or low voltage, either at fixed frequency, or at variable frequency, the frequency variation may be realized through static frequency converters.        an auxiliary network 17 called “emergency network” of type having direct current, whose main function is to compensate for a break down of the main auxiliary network and to allow maintaining in operation, at least temporarily, critical auxiliaries of the power plant in order to ensure the shutdown of the power plant and its restarting under correct conditions.        
From these critical auxiliaries, we can particularly cite the shaft line lubrication systems of the main unit 2, whose alternating current motors are rendered redundant by the direct current motors, supplied with power by the batteries, meant to ensure lubrication of the shaft line and its shutdown without damage in case of loss of supply of the main alternating current auxiliary network 4, 15.
Classically, the emergency network of an electricity production power plant is thus particularly constituted of one or more batteries and various equipment functioning on direct current, such as the distribution panels, control-command devices and motors and actuators. This equipment, powered on direct current, is connected to the main auxiliary network 4, 15, through the electronic circuits for conversion of alternating current into direct current. In normal operation of the power plant, the batteries are recharged from the main auxiliary network 15. In case of malfunction, the batteries restore the electric energy stored in the equipment operating on direct current.
However, the implementation within a same power plant of two types of auxiliary networks, a main auxiliary network 4, 15 in alternating current and a direct current network 17, induces complexity of implementation of the auxiliaries and engenders considerable costs throughout the lifecycle of the power plant (for example during study, commissioning and operation phases of the power plant). In fact, these two networks are partially redundant and the material operating on direct current, which particularly constitutes the emergency equipment of the critical auxiliaries, is less widespread than equivalent material operating on alternating current and, thus, in general more expensive. Furthermore, this emergency equipment and the elements constituting it, are exclusively dedicated to the emergency function in case of break down.
Thus, considering the network architecture of a classic power plant and the operating limitations relating to the maintenance of integrity of the power plant in case of break down of the supply of the main auxiliary network 4, 15, there is an actual need to develop a more efficient auxiliaries support architecture, which particularly allows simplifying the implementation of the latter and ensures their maintenance in operation even in case of break down, without necessarily having recourse to the emergency measures intended to be used only in this case.
In parallel, there is a need to provide an architecture allowing improving the network services which may be carried out through the power plants in order to make them more competitive, even extend the range of network services that may be ensured.